1. Technical Field
The disclosure generally relates to surface emitting lasers, and more particularly to a vertical cavity surface emitting laser (VCSEL), a VCSEL array device, an optical scanning apparatus incorporating such a VCSEL array device, and an image forming apparatus incorporating such an optical scanning apparatus.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser that emits light in a direction perpendicular to a substrate on which the laser is formed. The VCSEL provides high performance at lower cost than edge-emitting semiconductor lasers, particularly when highly integrated. For this reason, VCSELs are increasingly used as a light source of choice for optical communications, optical interconnections, optical pickups, and image forming apparatuses such as laser printers. For example, IEEE PHOTONICS TECHNOLOGY LETTERS, 1999, Vol. 11, No. 12, pp. 1539-1541 (“Non-Patent Document 1”) discloses a VCSEL that employs an AlGaAs material, wherein the laser has a single-mode output of 3 mW or more.
VCSELs for the aforementioned applications are required to have such characteristics as high active layer gain, low threshold current, high output, high reliability, and controlled polarization direction.
Typically, a VCSEL is formed by forming layers of semiconductor films on a GaAs substrate. Specifically, the VCSEL includes a cladding layer of AlGaAs formed on either side of a GaAs quantum well active layer, and reflecting mirrors (distributed Bragg reflectors (DBR)) made from alternate semiconductor layers of AlGaAs and AlAs films. A current confined layer is formed between the cladding layer and the reflecting mirror on the light emission side for improving performance.
However, in the VCSEL with the above structure, polarization control is difficult compared with the edge-emitting semiconductor lasers. In many cases, polarization control is dependent on unpredictable variations in the manufacturing process. Polarization may vary even on the same substrate, making it difficult to stably obtain VCSELs with constant polarization direction. It is difficult to achieve polarization stability because the VCSEL has a shorter cavity length and a larger opening for light emission than the edge-emitting laser.
When a VCSEL is used as a light source for forming an image by an image forming apparatus such as a laser printer, variations in polarization direction result in different reflectivities at a polygon mirror for optical scanning. As a result, optical utilization efficiency decreases or the image cannot be written stably. Various measures have been taken to stabilize polarization direction in VCSELs, as discussed below.
IEEE PHOTONICS TECHNOLOGY LETTERS, 1998, Vol. 10, No. 12, pp. 1676-1678 (“Non-Patent Document 2”) discloses a method for controlling polarization direction in a VCSEL whereby anisotropy of the substrate, for example, is utilized. Specifically, the method involves controlling the polarization direction to (−233) direction by using an inclined (311)B substrate.
Japanese Laid-Open Patent Application No. 2008-28424 (“Patent Document 1”) discloses a method for controlling polarization by providing anisotropy to a mesa structure in a VCSEL. Japanese Patent No. 3791193 (“Patent Document 2”) discloses a polarization control method utilizing the direction in which wire leads are drawn out. Japanese Laid-Open Patent Application No. 2008-16824 (“Patent Document 3”) discloses a polarization control method involving the application of a stress to the active layer by locally providing an oxidization region inside a VCSEL.
Japanese Laid-Open Patent Applications No. 11-340570 (“Patent Document 4”) and 11-354888 (“Patent Document 5”) disclose multi-beam semiconductor lasers in which, in order to meet the demand for higher speed in image forming apparatus such as laser printers, multiple light sources are provided on a single chip.
Japanese Laid-Open Patent Application No. 2002-217492 (“Patent Document 6”) discloses a method for forming an active layer on a substrate, wherein a relaxation layer is provided between the active layer and the substrate, the relaxation layer having an intermediate lattice constant between the active layer and the substrate, so that a high-quality active layer can be formed.
Japanese Laid-Open Patent Application No. 2003-347582 (“Patent Document 7”) discloses a method for forming a DBR on a substrate using AlInP/GaInP such that the lattice constant gradually varies, in order to form a high-quality active layer.
In the method according to Non-Patent Document 2, the substrate is inclined at 25°. As the inclination of the substrate increases, isotropic oxidation becomes increasingly difficult when forming an oxidized confined layer, thus leading to increased manufacturing difficulties. Further, such substrates are special substrates and are very expensive, making it difficult to manufacture the VCSEL at low cost.
In the method of Patent Document 1 the anisotropy of the mesa structure is reflected in the current confined region. In this case, current injection homogeneity may be lost, or a desired spot shape may not be obtained from the manufactured laser. In addition, in the method of Patent Document 1, the mesa structure anisotropy also affects the process of forming the current confined region in the VCSEL, so that uniform current injection is difficult.
In the method according to Patent Document 2, when the VCSEL is integrated at high density within a single chip, the intervals of the individual elements become narrower, thus making it difficult to form wires freely. If the intervals among the individual elements are increased for the sake of wiring, the area of the individual chip increases, resulting in an increased manufacturing cost.
In the method according to Patent Document 3, the structure of the VCSEL requires an additional manufacturing step, so that the manufacturing process becomes more complex and requires a longer manufacturing time, resulting in increased manufacturing cost. Further, the current confined layer is made of AlOx obtained by oxidizing AlAs, which produces a strain around the confined layer. As a result, dislocation is caused upon energization, resulting in a possible decrease in reliability. Thus, it is not preferable to provide a plurality of such current confined layers.
The multi-beam semiconductor lasers according to Patent Documents 4 and 5 are edge-emitting semiconductor lasers, wherein there is the problem of thermal interference between individual elements and it is difficult to obtain narrow pitches of several μm or less on the structure when the elements are arrayed one-dimensionally.
In the structure disclosed in Patent Document 6, the active layer has a greatly different lattice constant from the substrate. Although the relaxation layer with the intermediate lattice constant is provided between the substrate and the active layer, the presence of the layer with a large strain near the active layer strains the active layer, making it difficult to obtain high quality.
In the structure according to Patent Document 7, the lattice constant of the DBR between the substrate and the active layer is gradually changed in order to allow the formation of the active layer having a greatly different lattice constant from the substrate. Thus, the difference in lattice constant between the DBR near the active layer and the lattice constant of the active layer is smaller than in Patent Document 6. However, given the thickness of a DBR which is typically on the order of several μm, the stress applied to the active layer is large. Thus, it is difficult to obtain a high-quality active layer as in the case of Patent Document 6.
Thus, the conventional laser light sources cannot sufficiently cope with the increased speeds in image forming apparatuses such as, for example, laser printers.